1. Field of the Invention
The present invention relates to a power supply device such as a power supply for power tools and a backup power supply, and in particular to a power supply device that includes multiple battery modules that are serially connected to each other.
2. Description of the Related Art
Power supply devices such as a power supply for power tools and a backup power supply include multiple battery modules that are serially connected to each other in order to increase the output of the power supply devices. Thus, the power supply devices have high output. The reason is that the output of the power supply devices is proportional to the product of voltage and current of the battery modules. For example, in the case of a power tools that includes multiple battery modules that are serially connected to each other, the maximum output voltage is increased to 42 V, and in the case of a backup power supply that includes multiple batteries that are serially connected to each other, the maximum output voltage is increased to 57 V. In the case of nickel metal hydride battery power supply devices, the device includes battery modules each of which includes a plurality of nickel metal hydride batteries that are serially connected to each other. The battery modules are serially connected to each other. Thus, the nickel metal hydride battery power supply devices have high output. In the case of lithium-ion rechargeable battery power supply device, the device includes lithium-ion rechargeable batteries each of which serves as a battery module. A number of the lithium-ion rechargeable batteries as the battery modules are serially connected to each other. Thus, the lithium-ion rechargeable battery power supply devices have high output.
In power supply devices that include multiple battery modules that are serially connected to each other, the devices include a plurality of battery blocks. Battery modules are allocated to the plurality of battery blocks. The plurality of battery blocks are serially connected to each other at reference midpoints. In power supply devices that include multiple battery modules that are serially connected to each other, the devices detect the respective voltage values of the battery modules, and control charging/discharging operation. The reason is to prevent each serially-connected battery module from being overcharged and over-discharged. The electrical characteristics of batteries are likely to remarkably deteriorate if the batteries are overcharged and over-discharged. To prevent this adverse effect, a power supply device has been developed which includes circuitry that detects the respective voltage values of battery modules (see Japanese Patent Laid-Open Patent Publication No. 2003-282159). In the power supply device, battery modules that compose battery blocks are allocated to the battery modules.